How to use the ethereum-types.BlockParamLiteral.Pending function in ethereum-types

public async handleRequest(payload: JSONRPCRequestPayload, next: NextCallback, end: ErrorCallback): Promise {
        switch (payload.method) {
            case 'eth_getTransactionCount':
                const requestDefaultBlock = providerEngineUtils.blockTagForPayload(payload);
                if (requestDefaultBlock === BlockParamLiteral.Pending) {
                    const address = NonceTrackerSubprovider._determineAddress(payload);
                    const cachedResult = this._nonceCache[address];
                    if (cachedResult !== undefined) {
                        return end(null, cachedResult);
                    } else {
                        return next((requestError: Error | null, requestResult: any, cb: Callback) => {
                            if (requestError === null) {
                                this._nonceCache[address] = requestResult as string;
                            }
                            cb();
                        });
                    }
                } else {
                    return next();
                }
            case 'eth_sendRawTransaction':